Fire Hydrant

Background

A fire hydrant is an above-ground connection that provides access to a
water supply for the purpose of fighting fires. The water supply may be
pressurized, as in the case of hydrants connected to water mains buried in
the street, or unpressurized, as in the case of hydrants connected to
nearby ponds or cisterns. Every hydrant has one or more outlets to which a
fire hose may be connected. If the water supply is pressurized, the
hydrant will also have one or more valves to regulate the water flow. In
order to provide sufficient water for firefighting, hydrants are sized to
provide a minimum flowrate of about 250 gallons per minute (945 liters per
minute), although most hydrants can provide much more.

The need for fire hydrants developed with the advent of underground water
systems. Prior to that time, water was obtained from easily accessible
public wells or ponds. During the 1600s, London, England, began installing
an underground water system using hollowed-out logs as pipes. When there
was a fire, firefighters had to dig up the street and bore a hole in the
wooden pipes. Later wooden plugs were inserted into pre-drilled holes at
fixed intervals along the log pipes to make it easier for the
fire-fighters to get water. This gave rise to the term fire plug, which is
still sometimes used to refer to a hydrant.

As cities grew, so did their water systems. Larger systems meant increased
pressures, and cast iron pipes were laid to replace the rotting wooden
logs. When Philadelphia's new water system commenced operations in
1801, it not only served 63 houses and several breweries, but it also had
37 above-ground hydrants for fire protection. The first fire hydrant in
New York City was installed in 1817 by George Smith, who was a fireman. He
wisely located it in front of his own house on Frankfort Street.

Following the earthquake and fire that devastated San Francisco in 1906,
the city installed an extensive emergency water system that is still in
use. In addition to more than 7,500 hydrants connected to
standard-pressure water mains, the system includes a reservoir and two
tanks located on hills to supply nearly 1,400 high-pressure hydrants
throughout the city. There are also two salt-water pumping stations to
draw water from San Francisco Bay, plus five additional connections along
the waterfront to allow the city's fireboats to pump into the
hydrant system. As a final line of defense, the city has over 150
underground cisterns connected to unpressurized hydrants. Fire pumpers can
connect a rigid suction hose to these hydrants and pull the water out of
the cisterns by creating a vacuum.

Today, the size and location of fire hydrants in an area affect not only
the degree of fire protection, but also the fire insurance rates. In many
urban areas the lowly fire plug is all that stands between the first spark
and a multi-million-dollar fire loss.

Types of Hydrants

There are two types of pressurized fire hydrants: wet-barrel and
dry-barrel. In a wet-barrel design, the hydrant is connected directly to
the pressurized water source. The upper section, or barrel, of the hydrant
is always filled with water, and each outlet has
its own valve with a stem that sticks out the side of the barrel. In a
dry-barrel design, the hydrant is separated from the pressurized water
source by a main valve in the lower section of the hydrant below ground.
The upper section remains dry until the main valve is opened by means of a
long stem that extends up through the top, or bonnet, of the hydrant.
There are no valves on the outlets. Dry-barrel hydrants are usually used
where winter temperatures fall below 32° F (0° C) to prevent the
hydrant from freezing.

Unpressurized hydrants are always a drybarrel design. The upper section
does not fill with water until the fire pumper applies a vacuum.

Raw Materials

The hydrant barrel is usually molded in cast or ductile iron. Some iron
wet-barrel hydrants have an epoxy coating on the inner surface to prevent
corrosion. Other wet-barrel hydrants are molded in bronze. The hydrant
bonnet is usually made from the same material as the barrel. The valve
stem in a dry-barrel hydrant design is steel. The valve stems in a
wet-barrel hydrant are usually made from silicon bronze.

The hydrant outlets are molded in bronze. If the barrel is cast or ductile
iron, the bronze outlets are threaded into the barrel. If the barrel is
bronze, the outlets are cast as part of the barrel. The outlet caps may be
bronze, cast iron, or plastic.

Valve seats, seals, and gaskets are made from a variety of synthetic
rubbers including styrene butadiene, chloroprene, urethane, and butadiene
acrylonitrile. Fasteners may be zinc-plated steel or stainless steel.

Hydrants are given a coat of primer paint before they are shipped. When a
hydrant is installed, the outer surface is coated with an exterior-grade
paint.

Design

The basic design and construction of pressurized fire hydrants in the
United States are defined by the American Water Works Association (AWWA),
which sets general standards for hydrant size, operating pressure, number
of outlets, and other requirements. Unpressurized hydrants may be the same
design as the pressurized hydrants within a city or fire district in order
to maintain commonality, or they may be a simple capped pipe design with
no valves.

The main body of the hydrant is called the barrel or upper standpipe. It
may consist of a single piece or it may be made in two pieces. If it is
made in two pieces, the upper portion with the outlets is called the head
and the lower portion is called the spool. This terminology is not exact
and varies from one manufacturer to another, as well as from one city to
another.

The hydrant outlets usually have male National Standard Threads (NST) to
mate with fire hose couplings. The smaller outlets, sometimes called the
hose nozzles or connections, are 2.5-inch NST. The larger out-lets,
sometimes called the steamer nozzles or connections, are 4-inch or
4.5-inch NST. The outlet caps are secured to the hydrant body with short
lengths of chain. The terms hose connection and steamer connection date
back to the 1800s. Before the advent of modern fire apparatus, minor fires
were often fought by connecting a single hose line directly to the smaller
outlet on a pressurized hydrant. If the fire was larger, a steam-powered
pumper, called a steamer, took water from the larger hydrant outlet and
pumped it into several hose lines.

The hydrant valves are actuated by turning metal stems. The portion of
each stem that protrudes from the exterior of the hydrant is pentagonal
shaped and is called the operating nut. This five-sided nut requires a
special wrench to turn and helps prevent unauthorized use. On some
hydrants the operating nut is a separate piece that slips over the stem.
This allows the nut to be replaced if it becomes worn from use.

Some dry-barrel hydrants include a break-away feature to allow easy repair
if the hydrant is struck by a vehicle. This design includes a breaker ring
on the barrel of the hydrant near the ground and a breakable coupling on
the valve stem inside the hydrant. When struck, the upper barrel and stem
snap free without disturbing the under-ground piping or valve.

Although the basic components of all fire hydrants are similar, the shape
of hydrants

Fire hydrants are made through a process of metal casting. Once
manufactured, each hydrant is filled with water and pressurized to
twice the rated pressure to check for leaks.

varies from one manufacturer to another. Some hydrants have the classical
round body with a domed bonnet. Others have square or hexagonal bodies.
Some areas that are undergoing urban renewal have hydrants that are low
and modern looking.

The Manufacturing
Process

Making a fire hydrant is primarily a metal-casting process, and most
hydrant companies are metal foundries that specialize in manufacturing a
variety of municipal water works components.

Here is a typical sequence of operations for manufacturing a wet-barrel
fire hydrant.

Forming the molds

1 The outer surface of a mold is formed by a piece called the pattern.
To make a hydrant pattern, the hydrant's outer shape is generated
in three dimensions on a computer. This data is fed into a stereo
lithography machine, which uses laser beams to harden liquid plastic
into the shape of the hydrant. This hardened plastic piece is used to
make multiple copies of left and right pattern halves out of rigid
polyurethane.

2 The inner surface of a mold is formed by a piece called the core. To
make a hydrant core, the hydrant's inner shape is machined into
two halves of a block of aluminum or cast iron to form a cavity. The two
halves are clamped together, and the cavity is filled with a mixture of
sand and a plastic polymer. When the block of aluminum or cast iron is
heated gently, the polymer hardens the sand to form the core. The block
is then opened, and the core is removed. This process is repeated to
make multiple cores.

Casting the barrel

3 When a production run of hydrants is O ready to start, the patterns
and cores are brought to the mold-making machine. The left and right
patterns are pressed into the two halves of a mold filled with sand to
form impressions in the shape of the outer surface of the hydrant.
Molding sand is a special mixture that holds its shape without
crumbling. The hardened sand core is then carefully laid on its side and
held with short spacers to form a cavity between the core and the
impression in one of the mold halves. The other half of the mold is put
in place over the core and the mold is clamped together. This process is
repeated for each hydrant.

4 Molten metal is poured into each mold through an inlet passage called
a gate. Pouring continues until the metal starts to rise through outlet
on the opposite side called a riser. As the molten metal hardens, it
cooks the polymer in the core sand. This raises the temperature of the
polymer far beyond its initial setting point and causes it to break down
and allow the sand to become loose again.

Sideviews of a dry barrel and wet barrel hydrant.

5 After the casting has completely hardened, the mold is split apart and
the core sand is dumped out. The casting is placed in a horizontal
cylinder filled with small metal pellets and tumbled to remove any small
bits of metal or molding sand that may have adhered to the casting.

6 The cast gates and risers are cut off with an abrasive cut-off saw,
and are returned to the furnace. The cast barrel is ground with a
handheld power grinder to remove any rough surfaces.

7 If the hydrant has a two-piece barrel, the / head and spool are cast,
ground, and finished separately. If the hydrant is made from cast or
ductile iron, the outlets are cast, ground, and finished separately in
bronze.

Machining the barrel and valves

8 The entire hydrant is fixed lengthwise in a lathe, and shallow
concentric grooves are cut into the face of the lower flange. This
allows the flange to seal against a gasket when the hydrant is mounted.
The flange bolt holes may be drilled at this point or they may be
drilled just before shipment.

9 If the barrel is a two-piece design, the lower portion of the head has
National Pipe Taper (NPT) threads cut on the inside and the upper
portion of the spool has NPT threads cut on the outside to allow the two
pieces to be joined. The head is drilled and tapped on one side in the
area of the NPT threads to hold a locking set screw.

10 The hydrant—or the head, if it is a two-piece design—is
repositioned cross-ways in a lathe along the centerline of the larger
outlet. A rotating piece, called a fixture, clamps the hydrant in place
and provides a counterbalance as the hydrant is spun. The lathe bevels
the inner surface of the barrel around the outlet opening to provide a
smooth seating surface for the valve disc. The opening for the valve
stem insert is drilled and threaded. Finally the outlet or outlet
opening is threaded. This process is repeated for each of the outlets.

Assembling the hydrant

12 Starting with the upper valve, an oring seal is placed over the valve
stem, and the stem is threaded into the stem insert. The inside end of
the stem is pushed through the stem insert opening, and the disc holder,
rubber disc, and locking nuts are reached up inside the barrel, threaded
onto the stem, and locked in place with a set screw. The stem insert is
then threaded into the barrel, and the replaceable operating nut is
slipped over the outside end of the stem and held in place with a nut.
This process is repeated for each of the valves.

13 If the barrel is a two-piece design, an oring is slipped over the
threaded portion of the spool and the assembled head is screwed down to
seal against the oring. The threads are locked in place by a set screw.

Testing the hydrant

14 The AWWA standards require that bronze hydrants be rated at 150 psi
(1,034 kPa), and ductile iron hydrants be rated at 250 psi (1,723 kPa).
Each hydrant is filled with water and pressurized to twice the rated
pressure to check for leaks.

Preparing for shipment

15 After the hydrant is pressure tested, the outlet caps and chains are
attached, a plastic protector is slipped over the bottom flange, and the
exterior of the hydrant barrel is given a coat of primer paint.

Quality Control

All incoming material is inspected to ensure it meets the required
specifications. This includes spectrographic analysis of the raw materials
used to make the castings. The moisture content of the molding sand is
critical to the casting process, and it is checked before every casting
run. When a run of castings is machined, the first piece is checked for
proper dimensions before the remainder of the castings is machined.

The Future

It is unlikely that the fire hydrant will disappear from the urban
landscape anytime in the near future. Water is still the most
cost-effective fire suppressant, and the hydrant is still the most
cost-effective way to provide a ready supply of water. If anything, the
fire hydrant will gain importance as fire departments and taxpayers alike
realize that strategically placed, high-capacity hydrants can
significantly reduce fire insurance rates.

User Contributions:

Hi,
I would like to know about the weep hole in a hydrant. The term is use in the petrochemical industry in Trinidad and Tobago. When the hydrant is partially open water flows continously from under ground(verylight flow). When full open or fully closed the flow stops and everything is normal. One explanation I got was this is a design for cold countries to prevent the water from freezing in the supply line by having a constant flow. Can you explain this further and indicate the location of this weep hole on the hydrant.